Marina Silva, Brazil’s ex-environment minister, would replace former presidential candidate Eduardo Campos, who was killed in a plane crash on August 13.
Marina Silva is expected to be named in the next few days to run for president in October.
She agreed to run, but the decision still needs to be officially approved by Eduardo Campos’s Socialist Party (PSB).
The Socialists – the major partner in the opposition coalition established to run against President Dilma Rousseff – are expected to announce their decision after a meeting on August 20.
Marina Silva had decided to run for vice-president alongside Eduardo Campos after the Electoral Court in October 2013 refused to register her political movement, Rede Sustentabilidade (The Sustainability Network).
She fell out with the Workers’ Party government and left office to run against Dilma Rousseff in 2010.
An environmentalist and devout evangelical, Marina Silva polled surprisingly well, coming third with nearly 20% of the vote.
Dilma Rousseff, who is running for re-election in October, is the front-runner. On Sunday, the first opinion polls showing Marina Silva as candidate will be published in Brazil.
The first round of the presidential election will take place on 5 October and will go to a second round later that month if no candidate receives more than 50% of the votes.
The other main candidate is Aecio Neves, a current senator who will represent the Brazilian Social Democracy Party (PSDB).
Marina Silva is seen as a strong opponent and one who may win over even more support than Eduardo Campos would have.
She avoided addressing the issue as she travelled to Eduardo Campos’s funeral.
“I have a sense of responsibility and commitment,” she told journalists as she arrived in the north-eastern city of Recife.
Tens of thousands of people are expected to gather in the streets of the city on Sunday to pay their last respects to the popular politician.
Eduardo Campos came from a traditional family of politicians and was governor of Pernambuco for seven years, leaving office earlier this year with high approval rates.
His private jet – a Cessna 560XL – crashed in bad weather in the port city of Santos, in Sao Paulo state. It was travelling from Rio de Janeiro.
An investigation into the causes of the crash, which killed Eduardo Campos, 49, and six others, is under way.
The black box recovered from the wreckage of the plane did not record his flight, Brazilian Air Force (FAB) said.
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